求助一篇阅读题：国内题97年11月section B的长阅读

yuhaichao10UID: 3574819

Bracken fern has been spreading from its woodland strongholds for centuries, but the rate of encroachment into open countryside has lately increased alarmingly throughout northern and western Britain. A tough competitor, bracken reduces the value of grazing land by crowding out other vegetation. The fern is itself poisonous to livestock, and also encourages proliferation of sheep ticks, which not only attack sheep but also transmit diseases. No less important to some people are bracken’s effects on threatened habitats and on the use of uplands for recreational purposes, even though many appreciate its beauty.

Biological controls may be the only economic solution. One potentially cheap and self-sustaining method of halting the spread of bracken is to introduce natural enemies of the plant. Initially unrestrained by predators of their own, foreign predators are likely to be able to multiply rapidly and overwhelm intended targets. Because bracken occurs throughout the world, there is plenty of scope for this approach. Two candidates, both moths from the Southern Hemisphere, are now being studied.

Of course, biological control agents can safely be released only if it can be verified that they feed solely on the target weed. The screening tests have so far been fraught with difficulties. The first large shipment of moths succumbed to a disease. Growing enough bracken indoors is difficult, and the moths do not readily exploit cut stems. These are common problems with rearing insects for biological control.

Other problems can be foreseen. Policymakers need to consider many factors and opinions such as the cost of control compared to existing methods, and the impact of the clearance of bracken on the landscape, wildlife, and vegetation. In fact, scientists already have much of the information needed to assess the impact of biological control of bracken, but it is spread among many individuals, organizations, and government bodies. The potential gains for the environment are likely to outweigh the losses because few plants, insects, mammals, and birds live associated only with bracken, and many would benefit from a return of other vegetation or from a more diverse mosaic of habitats. But legal consequences of attempts at biological control present a potential minefield. For example, many rural tenants still have the right of “estoyers,” the right to cut bracken as bedding for livestock and uses. What would happen if they were deprived of these rights? Once a biological control agent is released, it is difficult to control its speed. What consideration is due landowners who do not want to control bracken? According to law, the release of the biological control agents must be authorized by the secretary of state for the environment. But Britain lacks the legal and administrative machinery to assemble evidence for and against release.

21.It can be inferred from the passage that it is advantageous to choose as the biological control agent a predator that is foreign to the targeted environment for which of the following reasons?
(A) Conservation groups prefer not to favor one native species over another.
(B) All local predators have already been overwhelmed by the target species.
(C) Local predators cannot be effectively screened since they already exist in the wild.
(D) There is little risk of an artificially introduced foreign predator multiplying out of control.
(E) Native predator species are generally limited by their own predators.

22.        It can be inferred from the passage that the screening tests performed on the biological control agent are designed primarily to determine
(A) its effectiveness in eliminating the target species
(B) the response of local residents to its introduction
(C) the risk it poses to species other than the target
(D) its resistance to the stress of shipment
(E) the likelihood of its survival indoors

23.        As it is discussed in the passage, the place of bracken within the forest habitat can best be described as
(A) rapidly expanding
(B) the subject of controversy
(C) well established
(D) circumscribed by numerous predators
(E) a significant nutrient source

21. 我没有找到解题线索

22. 定位至文中红色标识的地方，我选了答案E，同样没有找到答案C的线索

23. 没有找到线索


求热心版友解答，万分感谢~~~

yuhaichao10UID: 3574819

自顶一下！

cchen2014UID: 3590105

21题 定位第二段Initially unrestrained by predators of their own, foreign predators are likely to be able to multiply rapidly and overwhelm intended targets.
foreign的unrestrained by predators，所以Native 被 limited （restrain）by predators

22题 定位第三段Of course, biological control agents can safely be released only if it can be verified that they feed solely on the target weed.
目的就是为了保证 they feed solely on the target weed，也就是对其他物种没有威胁（the risk it poses to species other than the target）

23题我也没弄清，同问

yuhaichao10UID: 3574819

cchen2014 发表于 2015-5-18 11:08
21题 定位第二段Initially unrestrained by predators of their own, foreign predators are likely to be  ...

谢谢指教~

hijimoskaUID: 3781767

你好。虽然可能你已经不再考GRE了，但是可能会有很多人搜索这个令人困惑的题目。所以我还是想把自己认为比较合理的解释放上来，供大家参考一下。
文章第一句Bracken fern has been spreading from its woodland strongholds for centuries, but the rate of encroachment into open countryside has lately increased alarmingly throughout northern and western Britain.
它们是从这个woodland里面扩散出来的，说明这个woodland可能已经饱和了不再够他们spreading的！

hijimoskaUID: 3781767

hijimoska 发表于 2017-4-19 20:49
你好。虽然可能你已经不再考GRE了，但是可能会有很多人搜索这个令人困惑的题目。所以我还是想把自己认为比较 ...

所以，with in the forest habitat是被fern们well established. 已经被fern很好的建立了，它们还渴望建立新的基地，所以才会向着countryside。